1. Field
The removal of particulates from flue gases, particularly gases generated by the electrical power industry, has achieved considerable attention in recent years. One of the reasons for this attention is due primarily to the fact that government regulations prohibit substantial emission of particulates to the atmosphere.
Large particles are easily removed from the flue gas by mechanical devices; however, finer particles, e.g. fly ash, smaller than a few microns in size must be removed by other means. Electrostatic precipitators have been used to remove fly ash particulates but low sulfur fossil fuels do not respond well to fly ash removal by this method. To enhance efficiency of removal of fly ash from flue gas obtained from low sulfur coal, it has been customary to add sulfur trioxide in a proportion of from about 5-50 ppm, preferably 15-25 ppm, to the gas in order to enhance conductivity of the ash. 2. Description of the Prior Art
U.S. Pat. No. 2,602,734 discloses a method for recovering suspended material in gases from the flash roasting of sulfide ores such as zinc blende. The improved method comprises introducing sulfuric acid as a conditioning agent in a proportion of about 1.45 to about 2.9 grains (calculated as H.sub.2 SO.sub.4) per cubic foot of gas, directly into the roasting or flue gas and then precipitating the particulates by passing the gas through an electrostatic precipitator.
U.S. Pat. No. 2,746,563 discloses a method for treating flue gases by catalyzing the conversion of sulfur dioxide formed on burning of coal to sulfur trioxide and then precipitating the fly ash from the flue gas. Of course in this method the proportion of sulfur dioxide varies with the concentration of sulfur in the coal.
U.S. Pat. No. 3,689,213 discloses a method for treating flue gases by generating sulfur dioxide in the immediate vicinity of the point of use, burning the sulfur dioxide in excess air and forming a spiking gas, introducing the spiking gas into the flue gas, and then precipitating the particulates by electrostatic means. The example shows introducing a spiking gas having a sulfur trioxide concentration of about 5 per cent and a temperature of 520.degree. F into the flue gas.
U.S. Pat. No. 3,704,569 discloses a method for conditioning flue gas with sulfuric acid by forming a spiking gas having a temperature of about 435.degree. F and sulfuric acid concentration of about 2.8 percent, introducing the spiking gas containing vaporized sulfuric acid into the flue gas and precipitating the particulates by electrostatic means.
U.S. Pat. No. 3,581,463 discloses a process for removing particulates from a flue gas by diverting a small sidestream of flue gas, precipitating the particulates therein to prevent catalyst fouling, oxidizing the sulfur dioxide present in the sidestream in a catalytic converter to sulfur trioxide, and then reintroducing and mixing the sidestream containing sulfur trioxide with the flue gas prior to precipitation. The patentee reports that the sidestream coming from the boiler is about 800.degree. F and contains about 0.1 percent by volume sulfur dioxide. After conversion the sidestream containing sulfur trioxide is mixed with 300.degree. F flue gas.